1. Field of the Invention
The present invention relates to a method for ultrasonic wave transmission and an apparatus for ultrasonic wave transmission.
2. Description of Related Art
An ultrasonic sensor has been used for measuring a location of an obstacle existing forward of the ultrasonic sensor, and used for sensing a shape of the obstacle. Such an ultrasonic sensor includes a transmission element and a reception element. The transmission element transmits an ultrasonic wave, and the reception element receives and senses the ultrasonic wave reflected by an object. Thereby, the ultrasonic sensor measures, for example, a distance to the object located around the ultrasonic sensor.
This type of an ultrasonic sensor typically includes multiple sensor elements arranged in an array at given intervals. When the ultrasonic sensor measures a location of an obstacle and detects a shape of the obstacle, it is required to obtain a time interval between transmission and reception of the ultrasonic wave. Further, it is required to obtain a difference in time or phase between the ultrasonic wave received with one sensor element and that received with another sensor element. In the above case, the ultrasonic wave transmitted from the transmission elements can substantially reach to only an inside of a given area. The given area determines a detection area (i.e., angle, distance etc.) for detection of an obstacle. It may be preferable to transmit the ultrasonic wave with a directivity of the ultrasonic wave being controlled.
Japanese Unexamined Patent Application Publication Number 2004-125515 shows an ultrasonic sensor that is capable of transmitting an ultrasonic wave with the directivity thereof being controlled. More specifically, the ultrasonic sensor includes a reception element and multiple transmission elements, which are sources for transmitting the ultrasonic wave. When the multiple transmission elements transmit the ultrasonic wave, the ultrasonic sensor can scan a main beam by phase-shifting.
When the above method for transmitting an ultrasonic wave is employed, a large number of transmission elements are required to increase flexibility such as an increase in a directivity control range. Thus, a size of the ultrasonic sensor may be enlarged.